1. Field of the Invention
The present invention relates to a control technique of focusing a light beam on a data storage layer of a given optical disc.
2. Description of the Related Art
Various types of optical discs such as DVD-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R are already known as storage media on which a huge amount of information can be stored at a high density. In addition, brand-new types of optical discs, including Blu-ray Disc (BD), are currently under research and development in order to further increase the maximum densities and capacities of the optical discs.
An optical disc drive is used to read and/or write data from/on any of those various optical discs by focusing a light beam on the data storage layer of the optical disc. In processing an optical disc in which multiple data storage layers are stacked one upon the other, the optical disc drive can shift the focal point of the light beam from one of the data storage layers to another. Such shift of the focal point is called an “interlayer jump” or a “focus jump”.
For example, the optical disc drive disclosed in Japanese Laid-Open Publication No. 9-326123 (see paragraphs Nos. 0108 through 0118 and FIGS. 12 and 18) performs the focus jump in the following manner. Specifically, first, while performing no focus control operation (i.e., while putting the focus control operation on hold), the optical disc drive applies a pulse signal to an optical head. In response to the pulse signal applied, the optical head starts shifting the focal point. In this case, by controlling the pulse signal, the optical disc drive initially increases the shifting velocity of the focal point and then decreases it. Thereafter, by detecting the light beam that has been reflected from the optical disc, the optical disc drive determines whether the light beam is just in focus, or slightly out of focus, with the target data storage layer (i.e., whether the focal point has been formed right on the target data storage layer or just slightly passed it). The focus jump operation ends with this decision.
However, if an optical disc requires a high-precision focus control of the light beam, then the conventional optical disc drive might be unable to perform the focus jump operation thereon accurately. For example, a DVD, which should rotate at a velocity that is high enough to read and write data sufficiently fast, and a BD, on which data is stored even more densely than a DVD, require that the variations in the focal point fall within relatively narrow ranges. For that reason, the conventional optical disc drive might fail to control the focal point or perform the focus jump operation accurately enough.
What is worse, if the optical disc drive cannot perform the focus jump operation just as intended, the objective lens (i.e., the convergent lens) of the optical disc drive might come into contact with the optical disc unintentionally. For example, when a convergent lens with a numerical aperture (NA) of about 0.8 or more is used to write data on a BD or any other high-density optical disc, the distance between the optical disc and the convergent lens will be approximately 100 μm. In that case, while the focal point of the light beam is jumping from a relatively shallow data storage layer to a deeper one, a conventional low-precision optical disc drive could not prevent the convergent lens from contacting with the optical disc. As a result, the convergent lens and the optical disc both might get scratched in that case.